^^^ this and…
base fitness is developed over years, not 3 or 6 months. Now in my 7th year of training, my ftp has plateaued, something like a 10W gain over the last 2 years, but I’ve moved my ‘all day’ power up.
I haven’t tested with lactate but use 140-143bpm as my all-day power, here is an 8-hour HR paced effort: Calculating LT1 and LT2 approximately without a blood test? - #861 by WindWarrior
and along with some other long efforts and field tests, its a good proxy for the floor on long efforts where I’m pushing the pace but leaving something in the tank. Like what you need to do for an IM event where they say its won on the run.
and a couple of posts later showing the shift in power at that HR:
Being a little aggressive, I’m going to claim that is something like going from long steady rides at 77% IF / FTP up to 83% with very little increase if any increase in FTP. Training about 7-8 hours/week average, and 60-80% of that is endurance. And training age of 7 years. Your mileage may vary and all that.
I’m not a triathlete but if I was, once I’ve seen ftp plateau, my next frontier would be the floor as its going to deliver more than just long power (increased use of fat as fuel). To be honest I wish I would have started with the ‘go slow to go fast’ approach, but I didn’t have the patience or knowledge back in 2016.